On Wednesday, 9 August 2017 at 21:54:46 UTC, Q. Schroll wrote:
For a class/interface type `A` and a class `C` inheriting from `A` one can do

  A a = getA();
  if (auto c = cast(C) a)
  { .. use c .. }

to get a `C` view on `a` if it happens to be a `C`-instance.

Sometimes one cannot find a good new name for `c` while there is no advantage of accessing `a` when `c` is available. D does not allow to shadow `a` in the if-auto declaration for good reasons.

How often do you need this? I wouldn't go as far as saying downcasting is (always) evil, but it can be indicative of suboptimal abstractions [1].

How about relaxing the rule for cases like these, where the rhs is the lhs with a cast to derived?

  if (auto a = cast(C) a)
  { .. use a typed as C .. }

One can think of `a` being *statically* retyped to `C` as this is a (strictly) better type information. Internally, it would be a shadowing, but it does not matter as the disadvantages don't apply (if I didn't miss something).

While I can't see an obvious semantic issue, I would vote against such syntax because it introduces more special cases (and in this case an inconsistency w.r.t. variable shadowing) into the language and I don't see it providing enough of a benefit (downcasting should be used rarely) to justify that.

[1] http://codebetter.com/jeremymiller/2006/12/26/downcasting-is-a-code-smell/

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